Charge emission apparatus are used in a wide variety of different applications. For example, charge deposition print heads are used in electrostatic imaging apparatus to form an electrostatic latent image on a dielectric imaging surface by directing beams of charged particles onto the imaging surface. The electrostatic latent image is developed into a visible image using electrostatic toners or pigments, which are selectively attracted to the electrostatic latent image on the imaging surface. In such charge deposition print heads, an RF signal of up to several thousand volts is applied across a plurality of pairs of generator electrodes and discharge electrodes that are separated by an electrical insulator. The generator electrodes and the discharge electrodes typically are arranged orthogonally to one another. The applied signal creates localized charge source regions located at or near crossing points between the generator electrodes and the discharge electrodes. Electrical air gap breakdown between the discharge electrode and the electrical insulator generates electrical charge carriers that are emitted through apertures in the discharge electrodes and directed toward the imaging surface where the charges are deposited. The print heads are configured so that the charge deposited by each aperture forms a pixel or dot-like latent charge image on the imaging surface as it moves past the print head.
In general, there is a continuing push to increase the speed and the spatial resolution at which charge emission apparatus can deposit charge. Increasing the charge emission speed requires a linear increase in the required charging current. Increasing the spatial resolution, on the other hand, requires reducing the size of the discharge electrode aperture, which results in a concomitant decrease in the charging current. For high resolution electrostatic printing applications, an additional screen electrode aperture is needed to focus the charged particles. Such a screen electrode aperture imposes an additional decrease of up to 50% to 75% in the current depending on the extracting field. Attempts to increase charge deposition speed and spatial resolution by increasing the charging current oftentimes fail due to thermal failures and loss of reliability of the charge emitting printhead.
What are needed are improved charge-emission apparatus that are capable of emitting charged particles at a high rate and a high spatial resolution.